Within the packaging industry, use has long been made of packages of a single-use nature (so-called single-use disposable packages) for packing and transporting liquid foods. Conventional single-use packages are often produced from a flexible laminate material including a configurationally rigid but foldable core layer of paper or paperboard, or alternatively of a polymer, and outer layers of a liquid-tight polymer possessing superior thermosealing properties.
Such packaging containers are often produced in that a web of packaging laminate is reformed into a tube by the longitudinal edges of the web being united to one another, whereafter the tube is filled with the intended contents and thermosealed along narrow, transverse mutually spaced apart sealing zones. The sealed off portions of the tube containing the contents are then separated from the tube by means of incisions in the above-mentioned sealing zones and are formed, possibly by folding, into optional geometric configuration, depending upon how the sealing joints are oriented.
Alternatively, production of packaging containers may be put into effect in that a sheet-shaped blank of the packaging laminate is fold-formed and thermosealed into a container, whereafter the container is supplied with the intended contents and is sealed by means of fold formation and sealing of the upper region of the container.
Such a laminated packaging material is often supplemented with one or more additional layers of material possessing superior gas barrier properties in order to be able to be used also for packaging containers intended for food products which are sensitive to oxygen gas.
A common and efficient oxygen gas barrier material is aluminium which, in the form of a foil (Alifoil), may be disposed in a packaging laminate. Use of Alifoil entails, however, a number of disadvantages. Because of its slight flexibility, flexural and tensile cracks occur in the fold regions in a fold-formed package, as a result of which the packaging container is untight vis-a-vis penetrating oxygen gas. Moreover, Alifoil is difficult to handle on recycling or combustion or packaging materials, and consumed packaging containers are thereby less advantageous from the point of view of the environment.
In order to avoid the drawbacks inherent in Alifoil, oxygen gas barriers of polymer materials may be employed instead, such as, for example, ethylene vinyl alcohol (EVOH), or polyvinyl alcohol (PVOH) which are also to be preferred from the point of view of the environment, since they have relatively good properties as regards biological degradability.
However, it is a problem that the oxygen gas barrier properties in a layer of EVOH or PVOH are negatively affected by moisture and liquid and are thereby a relatively unreliable oxygen gas barrier in a packaging container intended for long transport, lengthy storage and cold storage of liquid foods.
According to Swedish Patent No. 440 519, a layer of polyvinyl alcohol emulsion is applied by means of coating direct onto one side of the core layer, which in this case consists of paper or paperboard, whereafter the applied aqueous polyvinyl alcohol layer is dried for the formation of a continuous, blanket layer. However, the raw, coarse surface of the paper core layer requires unnecessarily large application quantities of PVOH in order to achieve a blanket covering layer, at the same time as the PVOH layer is in direct contact with the moisture-sensitive paper or paperboard layer and is thereby not protected from the action of moisture via this layer.
According to Swedish Patent Application No. SE 95034948, these problems are obviated by laminating the moisture-sensitive oxygen gas barrier layer between two moisture-resistant material layers, disposed on the one side of the core layer, such as, for example, layers of extrudable thermoplastic, preferably polyethylene. The first moisture-resistant material layer between the core layer and the PVOH layer functions, on the one hand, as a smoothening layer, in which event the PVOH may be made thinner but nevertheless blanket covering, and, on the other hand, as a barrier against moisture penetration from the core layer. The second moisture-resistant material layer protects against the action of moisture from the other side of the PVOH layer.
There still remains the drawback inherent in a moisture-sensitive barrier layer in the region along the longitudinal edges of a packaging blank intended for the overlapping longitudinal joint seal. Normally, the longitudinal incision edges of the packaging blank are freely exposed in the longitudinal joints of a packaging container, for which reason the penetration of moisture via a porous, moisture-sensitive core layer or via the moisture-sensitive barrier layer proper may be able to take place, both in contact with the liqueform contents on the inside of the packaging container or in contact with moisture or a damp environment on the outside of the packaging container. In the area of the longitudinal joints of the packaging container, the gas barrier properties of the packaging material are thus still negatively influenced by surrounding moisture and damp, with a poor product protection during lengthy storage as a result.